Anything is possible, but easily accomplished? I doubt it. The boot image itself is just a pdf, of course, and while there are some ways to animate pdfs (make a postscript file that takes a LONG time to render because of numerous elements then convert it to pdf...creates an animation), none of them would be appropriate there (at least, none that I know of). Honestly, I don't see how you could do it without seriously hacking loginwindow or some other core part of the OS.